1. Field of the Invention
The present invention relates generally to electrical circuits, and more particularly but not exclusively to power on reset circuits.
2. Description of the Background Art
Power on reset circuits prevent invalid conditions in an electrical circuit by ensuring that the electrical circuit has sufficient power before allowing it to operate. In an integrated circuit chip, for example, a power on reset circuit may be employed to ensure that sufficient power supply voltage is available to the chip before the chip is enabled. The power on reset circuit enables the chip only when the power supply powering the chip is within a specified range. Otherwise, the power on reset circuit keeps the chip disabled. The voltage at which the power on reset circuit enables the chip is referred to as a “trip point.” The signal used to enable or disable the chip is also referred to as a “power on reset signal.”
FIG. 1 schematically shows an example power on reset circuit 100. Circuit 100 includes resistors R1, R2, R3, R4, and R5, capacitors C1 and C2, transistors M1 and M2, and an inverter 110. Circuit 100 is powered by a power supply labeled as VSUPPLY. VSUPPLY is typically the same power supply providing power to the chip controlled by circuit 100.
Resistors R1, R2, R3, and R4 form a resistor ladder that produce a scaled version of VSUPPLY. The scaled version of VSUPPLY appears on node vtrip, thereby controlling the voltage on the gate of transistor M1. When the scaled version of VSUPPLY reaches the threshold voltage of transistor M1, transistor M1 will turn ON, thereby pulling the input voltage to inverter 110 (see the node labeled as “vout”) to ground. This results in the output 112 of inverter 110 (see the node labeled as “por_xl”) to be at a logical HIGH state. A logical HIGH on output 112 enables the chip controlled by circuit 100. When VSUPPLY rises from ground to its operating level (e.g., after VSUPPLY has been turned OFF then ON), circuit 100 thus keeps the chip disabled until VSUPPLY rises to a level that is sufficient to turn ON transistor M1. The trip point of circuit 100 is the VSUPPLY voltage required to turn ON transistor M1.
The chip controlled by circuit 100 is disabled when output 112 is at a logical LOW. When the VSUPPLY voltage is below the trip point, transistor M1 is OFF and resistor R5 pulls the input voltage to inverter 110 to the VSUPPLY voltage. Inverter 110 will interpret that VSUPPLY voltage as a logical HIGH, thereby placing output 112 at a logical LOW. The logical LOW on output 112 serves as an active LOW reset signal, which resets the chip and keeps it disabled.
Capacitors C1 and C2 and transistor M2 help reduce the sensitivity of circuit 100 to noise. Capacitors C1 and C2 slow down the slew rate of nodes vtrip and vout, respectively, so that VSUPPLY has to exceed the trip point for a minimum amount of time before the voltage on VOUT crosses the threshold of inverter 110.
The VSUPPLY voltage below which transistor M1 turns OFF is referred to as a “falling trip point.” Transistor M2 provides some hysteresis by modifying the falling trip point of circuit 100. After VSUPPLY rises above the trip point, transistor M1 turns ON, which in turn turns ON transistor M2. This results in transistor M2 shorting out resistor R4, thereby lowering the VSUPPLY voltage required to keep transistor M1 ON and lowering the falling trip point at which M1 turns OFF.
A problem with circuit 100 is that its trip point may vary widely due to variations in the properties of its components, most importantly transistor M1. These variations in component properties (e.g., transistor threshold voltage) are typically caused by unavoidable variations in the manufacturing process and changes in the temperature of the chip. The chip is designed to work across an expected range of processes and temperature. For example, the trip point of circuit 100 may vary from 0.35 volts to 0.9 volts from circuit to circuit. This wide trip point variation may consume a considerable portion of the available voltage range where the chip is to be disabled and enabled. Also, at process corners where transistor M1 has a low threshold voltage, there may not be sufficient voltage to turn ON transistor M2 at the rising trip point (i.e., trip point crossed by a rising VSUPPLY), preventing hysteresis. This is specially problematic when circuit 100 is fabricated using low voltage processes (e.g., 1.2 volt or 1.5 volt processes).